Boreholes are drilled deep into the earth for many applications such as hydrocarbon exploration, geothermal production, and carbon dioxide sequestration. Different types of measurements are usually performed on a geologic formation in order to efficiently use production resources. One type of important measurement is permeability. Permeability relates to a measurement of the ability of a rock to transmit fluids generally through connected pores. Permeability is determined from the pore sizes that can transmit fluid and is usually measured using a nuclear magnetic resonance (NMR) tool disposed in a borehole penetrating the geologic formation
Carbonate is one type of geologic formation of interest. The pore system in carbonate formations is usually heterogeneous, yet such heterogeneity does not mean that different types of pores all contribute to fluid flow significantly. For example, in a micro-interparticle dual porosity formation, the fluid flows mainly through interparticle pores and the microporosity can be ignored in a permeability calculation. In an interparticle-vugg dual porosity formation, if the amount of vuggy porosity is not high, simulation can show that the permeability is not altered that much because of the presence of vuggy porosity.
In order to treat different types of pores differently, it is necessary to do pore typing. Typically, in prior art NMR logging interpretation, pore typing is done with NMR relaxation time cut-off values applied universally. The universally applied cut-off values divided the pores into macro-, meso-, and micro-sized pores. Coates and SDR equations are adapted to a hybrid set of equations such that one equation covers the high permeability part of the formation and the other equation covers the low permeability part of the formation. Unfortunately, pore typing based on universally applied sizes has limitations in resolving the true carbonate characteristics in complex carbonate formations, each with a distinct set of pores present. By relying on universally applied pore sizes, the prior art NMR logging interpretation may fail to predict permeability accurately. Hence, it would be appreciated in the drilling industry if techniques to determine formation permeability could be improved.